1. Field of the Invention
The present invention relates generally to clamp mechanisms for injection molding machines and more particularly to the frame structure and die height adjustment mechanism for injection molding machines without tie bars between platens.
2. Description of the Related Art
Injection molding machines have traditionally been designed to use hydraulic systems as their primary source of motive power. Although the hydraulics in molding machines have proven to be satisfactory over the years, such systems do have several inherent disadvantages. The hydraulic oil is subject to dirt and contamination in an industrial environment and requires filtering and maintenance. Further, there is the potential for oil leakage making them unsuitable for "clean room" manufacturing environments, and possibly contaminating the soil and groundwater over time. The hydraulic drive has limited positioning accuracy and repeatability, and changes in temperature of the hydraulic oil will lead to further variations in performance. Finally, the hydraulic drive is not energy efficient, and therefore, it requires heat exchangers and coolers to remove heat from the oil in order to maintain a constant oil temperature for stable performance.
Electric motor servomechanism drives provide an alternative to hydraulics and have been available for many years. With the recent advances in motor technology, and improvements in clamp mechanisms, there has been increasing interest in molding machines which are completely powered by electric motors (all-electric machines). Combining the state of the art in electric motors with reliable mechanical drive systems provides a machine that is clean, quiet, precise, energy efficient and has repeatable performance.
There has also been increasing interest in providing easier access to the mold mounting surfaces of an injection molding machine. In addition to the size of the mold itself, there are typically cooling lines, core mechanisms and other ancillary devices attached to the mold body. In a conventional molding machine (with tie bars), all or part of the attached equipment may have to be removed and/or the mold will have to be manipulated to fit between the tie bars so that it can be attached to the mounting surface. One solution, particularly on the smaller tonnage machines, has been to forgo the use of tie bars between platens and rely on the machine frame to provide the structure required to withstand the forces generated during the molding process. For convenience, this type of molding machine will be referred to herein as an "open frame" injection molding machine.
Although the open frame machines have found some measure of success, there have been certain drawbacks. Most noticeably, angular deflection of the "stationary" mold mounting surface occurs when the desired tonnage is applied to hold the mold closed during injection; such deflection can result in unsatisfactory parts production and excessive mold wear. Prior art open frame machines have attempted to compensate for this deflection by providing means to pivot slightly one or both of the mold mounting surfaces. However, this approach has not been completely satisfactory since the performance remains inconsistent due to the difficulties associated with aligning the mold sections and maintaining uniform pressure on the mold. Uniform pressure is particularly important to avoid localized "flash" at the parting line and other part quality variations, such as part weight, in multiple cavity molds. Prior art systems also tend to generate significant wear of the mold surfaces and alignment pins, resulting in more frequent mold maintenance.
A further drawback to the open frame injection molding machine involves the difficulties associated with making the machine all-electric. For practical purposes, an all-electric molding machine must take advantage of the mechanical efficiencies of a toggle mechanism to achieve the required clamping forces and open/close the mold quickly. However, since a toggle mechanism has a fixed length of stroke, a die height adjustment mechanism must also be provided to accommodate molds of varying size. This presents a difficulty on the open frame machine since the more effective die height mechanisms have typically been combined or associated with the tie bars and rear platen in machines with the toggle type clamp mechanisms. In addition, prior art mechanisms are configured in a way that increases the overall length of the machine; this adds to the cost of the machine and reduces the inherent ability of the frame to maintain proper platen alignment. Accordingly, there is a need for an efficient die height adjustment mechanism for a toggle type (all-electric), open frame injection molding machine.